Diabetes mellitus is a metabolic disorder in which the ability to utilize glucose is partly or completely lost. In the treatment of diabetes mellitus, many varieties of insulin formulations have been suggested and used, such as regular insulin, isophane insulin (designated NPH), insulin zinc suspensions (such as Semilente®, Lente®, and Ultralente®), and biphasic isophane insulin.
Human insulin consists of two polypeptide chains, the A and B chains which contain 21 and 30 amino acid residues, respectively. The A and B chains are interconnected by two disulfide bridges. Insulin from most other species is similar, but may contain amino acid substitutions in some positions. Within the last decade a number of human insulin analogues have been developed. They are designed for particular profiles of action, i.e. fast acting or prolonged action. Commercially available products comprising such insulin analogues include Levemir®, NovoRapid®, Humalog®, Apidra® and Lantus®.
Human insulin is rapidly degraded in the lumen of gastrointestinal tract by the action of multiple proteases limiting its absorption into circulation. Insulin analogues that are hydrophilic and stabilized towards proteolytic degradation show higher bioavailability in animal models when compared to native insulin.
Incorporation of disulfide bonds into proteins is one of nature's ways of improving protein stability; a correlation between the abundance of disulfide bonds and the maximum growth temperature among thermophilic organisms has been found, implicating the importance of disulfide bonds in protein stabilization in high temperature environments (Mallick P, et al, 2002, Proc. Natl. Acad. Sci. USA, 99, 9679-9684; Ladenstein R, et al, 2006, FEBS J., 273, 4170-4185). There are also many examples of disulfide bonds being successfully engineered into proteins with concomitant increases in stability. One of the largest stabilizations was achieved for RNAse barnase (upto 4.1 kcal mol−1 depending on the position of the new disulfide bond) (Clarke J., Fersht A., 1993, Biochem., 32, 4322-4329). Such stabilization is brought about by increasing the activation energy required for unfolding or by constraining the unfolded conformations of the protein and thereby decreasing their conformational entropy (Pace C. N., 1990, Trends Biol. Sci., 14-17). However, much more needs to be learned in this emerging area of research. To date, there are no reports of engineered disulfide bonds in insulin.
There is still a need for novel insulin analogues which are stable.